A scanning charged particle beam imaging system uses a charged particle beam to scan the surface of a sample, collects the secondary electrons emitted from the sample to form images of the sample. The system has to focus the charged particle beam on the surface of the sample when sampling. However, the initial surface charging on the sample surface and/or variation of surface charging may alter the focus of charged particle beam probe for image sampling, or the surface charging induced by previous image sampling becomes severe enough to distort the subsequent image. As an attempt to solve these problems, pre-scan and post-scan methods have been developed for regulating the surface charging.
The pre-scan method includes regulating the surface charging and then sampling the sample, wherein the purpose of regulation of surface charging is to achieve a neutral or uniform surface charging. That improves the secondary electron image contrast.
The post-scan method includes sampling the sample and then regulating the surface charging, wherein the purpose of regulation of surface charging is to release or reduce the surface charging induced by previous image sampling to an extent where the quality of a subsequent image sampling will not be severely harmed.
One of the known approaches proposed to control the surface charging problem searches for the optimal beam condition for the imaging scan. The optimal beam condition is determined in a sense that it induces no or little surface charging at the given the characteristics of the sample to be inspected (e.g. composing materials, physical/electrical properties thereof, etc.) and the required imaging specifications. The found optimal beam condition can then be used for sampling the sample with confidence of no or tolerable surface charging in terms of distortion or uniform focus, but may not guarantee the image quality in term of image topographical contrast and voltage contrast.
Another known method makes use of an additional charged particle beam probe source, which is typically referred to as a flood gun, to scan the imaging area beforehand so as to regulate its surface charging condition thereby making it suitable for the subsequent imaging scan. For example, the flood scan may be performed on the entire wafer. The drawback of this method is that it is hard to control the surface charging uniform and hold the charging long enough for some samples to be inspected. For example, when the imaging area is large, it may take hours to finish sampling the whole imaging area. As a result, for the end portions of the imaging area, the regulated charging condition may have already changed when the imaging scan is finally made to these regions.
Accordingly, the present invention is intended to propose a method to more efficiently carry out surface charging regulation during charged particle beam imaging.